Issue 47

D. Rigon et alii, Frattura ed Integrità Strutturale, 47 (2019) 334-347; DOI: 10.3221/IGF-ESIS.47.25 336 [16]. Depending on the severity of the stress concentration effect, the heat energy has been evaluated at a point or it has been averaged in a material volume. More precisely, in case of blunt notches, the specific heat loss has been evaluated at a point of a specimen (Q), i.e. at the notch tip; in case of severe V-notches or cracks, the specific heat loss has been averaged in a control volume of material surrounding the tip of the stress raiser (Q*) as proposed in [20,21]. In [16,18], thermocouples having wires of 0.127 mm in diameter were attached at the notch tip, by using 1.5-2 mm silver- loaded glue dot diameter. Conversely, in [19,22] temperature was monitored by means of an infrared camera, because of the much more localized temperature field caused by the notch tip radii lower than 3 mm, which had not been tested in [16,18]. In this paper, an automatic data processing was developed in the Matlab® code to investigate the distribution of the energy dissipated around the notch tip. Such a procedure was applied to the fatigue test results published in [22], which are synthesised in Fig. 1. Taking into account individual fatigue tests, a steady-state finite element analysis was performed assigning the distribution of specific heat power generation coming from the Q maps. Then, the numerical temperature results were compared to the experimental ones. M ATERIALS AND METHODS Experimental protocol pecimens characterised by three different notch tip radii r n , namely 3, 1 and 0.5 mm, were machined from a 4-thick- mm hot rolled AISI 304L stainless steel sheet, according to the geometry shown in Fig. 2a. The density and specific heat of the material are equal to 7940 kg/m 3 and 507 J/(kg K), respectively. A FLIR SC7600 infrared camera has been adopted for recording the T(x,y) temperature maps. In order to synchronize the force and temperature signals, the infrared camera was equipped with an analog input interface. In addition, a spacer ring was used to achieve a spatial resolution around 20 μm/pixel. The frame dimension of 320x256 pixels has been set and the notch was positioned in the center of the frame to exclude the temperature error caused by vignetting. To monitor the evolution of Q during each test, 1 to 10 temperature acquisitions (before crack initiation) were performed using a 10-second- long sampling window with f acq =200 Hz (2000 frames), starting from a time t s . The time window consists of approximately 5 seconds of running test (i.e. 1000 frames between t s and t*, Fig. 3a), followed by the machine stop at the time t*, and the remaining 5 seconds of acquisition to capture the cooling gradient (i.e. additional 1000 frames after t*). With the aim to increase the material emissivity, one specimen’s surface was black painted. On the contrary, the opposite surface was polished to detect the possible presence of a fatigue crack emanating from the notch tip, by using a AM4115ZT Dino-lite digital microscope (with a magnification ranging from 20x to 220x). Figure 3 : Schematic illustration of the time-variant temperature for one pixel (a) and an example of evaluation of the t* after a fatigue test stop. (b) Data post-processing The acquired temperature maps and force signal were post-processed by means of the ALTAIR 5.90.002 commercial software and saved as Altair PTW film file (*.ptw). Since the test machine takes some tenth of a second to definitively stop, an engineering definition of t* was introduced, as follows. The time t* was defined taking advantage of the data post- processing carried out by using the ALTAIR 5.90.002 commercial software and it was defined as the time when the first S